1. Field of the Invention
The present invention relates to a method for cleaning semiconductor wafer and an apparatus for the same, and more particularly, to a wet wafer cleaning method and apparatus for preventing semiconductor wafers from being eroded by a cleaning liquid.
2. Description of the Related Art
A typical way of cleaning the semiconductor wafer is a dip-type cleaning. In the dip-type cleaning, a plurality of wafers is transferred by a wafer transfer apparatus, using or without using a carrier cassette, to a cleaning bath and dipped in a cleaning liquid in the cleaning bath so that the wafers are simultaneously cleaned.
However, as technology migrates to the deep submicron generation, semiconductor devices are more highly integrated and their patterns are fine pitched. Accordingly, the surface of the wafer is required to be cleaned to have a higher level of cleanliness. As a wet wafer cleaning method meeting the high level of cleanliness, suggested is a sheet-type wet cleaning method in which wafers are cleaned sheet by sheet in a closed cleaning housing.
During cleaning of only an upper (front) surface of a wafer, it happens that a cleaning liquid supplied to the upper surface of the wafer wraps around a lower surface of the wafer and contaminates the lower surface of the wafer. Once the lower surface of the wafer is contaminated, a jig for handling the wafers, such as a robot hand of a scalar-type robot becomes also contaminated due to the contaminants on the lower surface of the wafer. Thus, other wafers are continuously contaminated by the contaminated robot handle.
Accordingly, a cleaning technology to prevent the wrap-around of a cleaning liquid is strongly needed. Several approaches to prevent the cleaning liquid wrap-around are known in the art.
For example, a wrap-around prevention technology is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 7-183265 (paragraph 0023 and FIG. 2). The disclosed method simultaneously cleans upper and lower surfaces of a wafer at a time to prevent the wrap-around of the cleaning liquid. In more detail, while a wafer chuck holds an outer edge (periphery) portion of the wafer, a cleaning liquid is supplied to the upper and lower surfaces of the wafer from nozzles disposed above and under the wafer, respectively. Then, the cleaning liquid spreads out on the upper and lower surfaces of the wafer in a direction from the center to an outer edge portion of the wafer and further from the edge portion to the outside (atmosphere), by the centrifugal force generated upon the rotation of the wafer. Accordingly, wrap-around of the cleaning liquid is prevented.
Another wrap-around prevention technology is disclosed in Japanese Patent Application Kokai No. 2003-7664 (paragraph 0055, FIGS. 2 and 6).Pure water is sprayed to a lower surface of a wafer from a lower nozzle disposed under the wafer while a cleaning liquid is supplied onto an upper surface of the wafer from a supply nozzle disposed above the wafer to prevent the cleaning liquid supplied to the upper surface of the wafer from flowing to the lower surface. The cleaning liquid supplied onto the upper surface of the wafer flows from a center portion of the wafer to a circumferential edge portion of the wafer along the upper surface of the wafer by a centrifugal force generated due to the rotation of the wafer, and the pure water supplied to the lower surface of the wafer also flows from a center portion of the wafer toward the circumferential edge portion of the wafer along the lower surface by the centrifugal force. Accordingly, movement of the cleaning liquid to wrap-around the lower surface of the wafer is stopped by the pure water on the lower surface. The pure water on the lower surface of the wafer serves as the sealing against the cleaning liquid.
In principle, the above described two conventional wrap-around prevention methods can prevent the cleaning liquid from flowing to the lower surface of the wafer. In reality, however, whether the wrap-around of the cleaning liquid is prevented or not depends upon a revolution speed (rpm) of the wafer and a viscosity of the cleaning liquid. If the wafer rotates at a high speed and a viscosity of the cleaning liquid is low, the cleaning liquid flies off the circumference of the wafer by a centrifugal force. In this case, there may be no need to use the above described prevention method(s) to prevent the wrap-around of cleaning liquid. On the other hand, if the wafer rotates at a low speed and a viscosity of the cleaning liquid is high, one of the above described wrap-around prevention methods is needed.
In any cases above, however, a circumferential outer edge surface region of an upper surface of the wafer disposed on a stage can be eroded by a cleaning liquid. After the erosion, the circumferential outer edge of the wafer becomes sometimes tapered. Certain types of wafers would have no problems even when the outer circumferential edge region of the wafers is exposed to the cleaning liquid and thus eroded by the cleaning liquid, but other types of wafers would have problems if the outer circumferential edge portion is eroded. In the later cases, the above described two prior art methods are not satisfactory and would cause a serious problem. Accordingly, a wafer cleaning method which does not cause any damage to a circumferential outer edge surface region of the wafer is needed.